Multidrug resistance (MDR) encountered during therapeutic treatment is a fatal occurrence for cancer patients, Consequently, understanding the mechanisms involved in this clinical event has become a major goal for many scientific groups. By studying DNA hypermethylation in a human MDR cell line, MCFTIAdrR, we have discovered the WTH3 gene, a novel member of the Rab6 gene family. Rab6 is a ras-like GTPase considered to stimulate intracellular retrograde transport (Golgi to endoplasmic reticulum). Introducing the WTH3 gene into MDR cell lines reversed the MDR phenotype and down regulated the expression level of MDR1. Therefore, WTH3 appears to function as a negative regulator involved in MDR development. Two mechanisms are possibly involved in such activity: 1) WTH3 works on an intercellular trafficking pathway to influence MDR and 2). WTH3 works on a transcription regulation pathway of the MDR1 gene. To test this hypothesis our experimental designs are: 1) to study the WTH3 gene's role in intercellular trafficking pathways and MDR phenotype development by; defining its subcellular location by tagged peptide markers, fused fluorescent proteins, or its antibodies. Analyzing WTH3's effects on secretion and retrograde transport pathways by using protein markers to perform transient transfection assays. Testing the host cells' drug sensitivity and retention using inducible stable transfection assays. Determining the effects of WTH3 on the MDR phenotype by anti-sense modulation strategies. 2) to study the WTH3 gene's role on the MDR1 gene expression regulation pathway by; testing WTH3's effect on wild type, deleted and mutated MDRI promoter constructs using luciferase as a reporter gene. Examining WTH3"s effect on three known positive MDR1 transcription regulators (mutated p53, rat, and SP1) for their activation of the MDR1 promoter/reporter system by co-expression approaches. Investigating WTH3's role on regulating MDR1 gene expression and the MDR phenotype in primary cultured breast tumor cells. Since the hypothesis and experimental designs are based on significant preliminary data and existing information, we are confident that our research wilt elucidate novel mechanisms involved in MDR, which could lead to the rational design of more effective treatment strategies to circumvent drug resistance.